ScrollFillText

npx motionfoundry add scroll-fill-text

1import { ScrollFillText } from "@motionfoundry/ui";2 3export function Example() {4  return (5    <ScrollFillText />6  );7}

1"use client";2 3import * as React from "react";4import styles from "../styles/scroll-fill-text.module.css";5 6interface SmoothScrollState {7  /** Current scrollTop of the viewport in px */8  scroll: number;9  /** Maximum scrollTop in px */10  limit: number;11  /** scroll / limit, 0..1 */12  progress: number;13  /** Signed scroll velocity in px per 60fps frame */14  velocity: number;15}16 17interface SmoothScrollOptions {18  lerp: number;19  wheelMultiplier: number;20  smooth: boolean;21  onScroll?: (state: SmoothScrollState) => void;22}23 24const clamp = (v: number, min: number, max: number) => Math.min(max, Math.max(min, v));25 26function usePrefersReducedMotion(): boolean {27  const [reduced, setReduced] = React.useState(false);28  React.useEffect(() => {29    const mq = window.matchMedia("(prefers-reduced-motion: reduce)");30    const update = () => setReduced(mq.matches);31    update();32    mq.addEventListener("change", update);33    return () => mq.removeEventListener("change", update);34  }, []);35  return reduced;36}37 38/** True when a nested scrollable element under the pointer can still scroll in the wheel direction. */39function nestedCanScroll(node: EventTarget | null, root: HTMLElement, dy: number): boolean {40  let el: Element | null = node instanceof Element ? node : null;41  while (el && el !== root) {42    if (el instanceof HTMLElement) {43      if (el.dataset.mfScrollPrevent !== undefined) return true;44      const overflowY = window.getComputedStyle(el).overflowY;45      if ((overflowY === "auto" || overflowY === "scroll") && el.scrollHeight > el.clientHeight + 1) {46        const canMove = dy < 0 ? el.scrollTop > 0 : el.scrollTop + el.clientHeight < el.scrollHeight - 1;47        if (canMove) return true;48      }49    }50    el = el.parentElement;51  }52  return false;53}54 55/**56 * Self-contained smooth scroller for an overflow container. Wheel input updates a target57 * position; a rAF loop eases the real scrollTop toward it (frame-rate independent lerp) and58 * stops once settled. Touch, keyboard and scrollbar drags stay native. At the top/bottom edge59 * wheel events are released so the page keeps scrolling.60 */61function useSmoothScroll(62  containerRef: React.RefObject<HTMLElement | null>,63  { lerp, wheelMultiplier, smooth, onScroll }: SmoothScrollOptions,64): React.RefObject<SmoothScrollState> {65  const stateRef = React.useRef<SmoothScrollState>({ scroll: 0, limit: 0, progress: 0, velocity: 0 });66  const callbackRef = React.useRef(onScroll);67  React.useEffect(() => {68    callbackRef.current = onScroll;69  }, [onScroll]);70 71  React.useEffect(() => {72    const el = containerRef.current;73    if (!el) return;74    const ease = clamp(Number.isFinite(lerp) ? lerp : 0.1, 0.01, 1);75    const multiplier = Number.isFinite(wheelMultiplier) ? wheelMultiplier : 1;76    let target = el.scrollTop;77    let current = target;78    let raf = 0;79    let lastTime = 0;80    let lastNative = el.scrollTop;81    let lastNativeTime = 0;82    let settleTimer: ReturnType<typeof setTimeout> | undefined;83 84    const getLimit = () => Math.max(0, el.scrollHeight - el.clientHeight);85 86    const emit = (velocity: number) => {87      const limit = getLimit();88      const scroll = clamp(el.scrollTop, 0, limit);89      lastNative = el.scrollTop;90      const s = stateRef.current;91      s.scroll = scroll;92      s.limit = limit;93      s.progress = limit > 0 ? scroll / limit : 0;94      s.velocity = velocity;95      callbackRef.current?.(s);96    };97 98    const tick = (time: number) => {99      const dt = lastTime ? Math.min(64, Math.max(1, time - lastTime)) : 16.667;100      lastTime = time;101      const limit = getLimit();102      target = clamp(target, 0, limit);103      if (Math.abs(el.scrollTop - current) > 2) {104        // Something else moved the viewport (scrollbar drag, keyboard, touch): hand control back.105        current = target = el.scrollTop;106        raf = 0;107        lastTime = 0;108        emit(0);109        return;110      }111      const prev = current;112      const frames = dt / 16.667;113      current += (target - current) * (1 - Math.pow(1 - ease, frames));114      if (Math.abs(target - current) < 0.35) current = target;115      el.scrollTop = current;116      emit((current - prev) / frames);117      if (current === target) {118        raf = 0;119        lastTime = 0;120        emit(0);121        return;122      }123      raf = requestAnimationFrame(tick);124    };125 126    const onWheel = (event: WheelEvent) => {127      if (event.ctrlKey || event.defaultPrevented) return;128      if (Math.abs(event.deltaX) > Math.abs(event.deltaY)) return;129      let dy = event.deltaY;130      if (event.deltaMode === 1) dy *= 16;131      else if (event.deltaMode === 2) dy *= el.clientHeight;132      dy *= multiplier;133      if (dy === 0 || nestedCanScroll(event.target, el, dy)) return;134      const limit = getLimit();135      if (!raf) current = target = el.scrollTop;136      // Release to native scrolling at the edges so the page is never trapped.137      if ((dy < 0 && target <= 0) || (dy > 0 && target >= limit - 0.5)) return;138      event.preventDefault();139      target = clamp(target + dy, 0, limit);140      if (!raf) {141        lastTime = 0;142        raf = requestAnimationFrame(tick);143      }144    };145 146    const onNativeScroll = () => {147      if (raf) return;148      const top = el.scrollTop;149      const delta = top - lastNative;150      if (delta === 0) return;151      const now = performance.now();152      const dt = lastNativeTime ? clamp(now - lastNativeTime, 8, 100) : 16.667;153      lastNativeTime = now;154      current = target = top;155      emit(delta / (dt / 16.667));156      if (settleTimer) clearTimeout(settleTimer);157      settleTimer = setTimeout(() => {158        lastNativeTime = 0;159        emit(0);160      }, 90);161    };162 163    if (smooth) el.addEventListener("wheel", onWheel, { passive: false });164    el.addEventListener("scroll", onNativeScroll, { passive: true });165    const ro = new ResizeObserver(() => emit(stateRef.current.velocity));166    ro.observe(el);167    if (el.firstElementChild) ro.observe(el.firstElementChild);168    emit(0);169 170    return () => {171      if (smooth) el.removeEventListener("wheel", onWheel);172      el.removeEventListener("scroll", onNativeScroll);173      ro.disconnect();174      if (raf) cancelAnimationFrame(raf);175      if (settleTimer) clearTimeout(settleTimer);176    };177  }, [containerRef, lerp, wheelMultiplier, smooth]);178 179  return stateRef;180}181 182export interface ScrollFillTextProps {183  /** Paragraph that fills word by word */184  text?: string;185  /** Color of words that have not been reached yet */186  dimColor?: string;187  /** Color of filled words */188  fillColor?: string;189  /** Maximum font size in px (shrinks on narrow containers) */190  fontSize?: number;191  /** Small label shown above the paragraph */192  eyebrow?: string;193  /** Scroll distance used for the fill, in viewport heights */194  length?: number;195  /** Viewport height in px */196  height?: number;197  /** Viewport corner radius in px */198  borderRadius?: number;199  /** Interpolation factor per 60fps frame (0.05 = floaty, 0.3 = snappy) */200  lerp?: number;201  /** Multiplier applied to wheel / trackpad deltas */202  wheelMultiplier?: number;203  /** Enable smooth wheel scrolling (native scrolling when false or under reduced motion) */204  smooth?: boolean;205  /** Optional className merged onto the root */206  className?: string;207  /** Inline styles merged onto the root */208  style?: React.CSSProperties;209}210 211const DEFAULT_TEXT =212  "Great motion is never decoration. It is a quiet guide that tells you where to look, what just changed, and what comes next. Scroll slowly and watch each word arrive exactly when you do.";213 214export function ScrollFillText({215  text = DEFAULT_TEXT,216  dimColor = "#3f3f46",217  fillColor = "#f4f4f6",218  fontSize = 30,219  eyebrow = "Scroll to read",220  length = 2.5,221  height = 380,222  borderRadius = 20,223  lerp = 0.1,224  wheelMultiplier = 1,225  smooth = true,226  className = "",227  style,228}: ScrollFillTextProps) {229  const reduced = usePrefersReducedMotion();230  const rootRef = React.useRef<HTMLDivElement>(null);231  const viewportRef = React.useRef<HTMLDivElement>(null);232 233  const handleScroll = React.useCallback((state: SmoothScrollState) => {234    const root = rootRef.current;235    if (!root) return;236    const fill = clamp((state.progress - 0.04) / 0.82, 0, 1);237    root.style.setProperty("--mf-sft-p", fill.toFixed(4));238    root.style.setProperty("--mf-sft-hint", clamp(1 - state.progress * 6, 0, 1).toFixed(3));239  }, []);240 241  useSmoothScroll(viewportRef, { lerp, wheelMultiplier, smooth: smooth && !reduced, onScroll: handleScroll });242 243  const words = text.split(/\s+/).filter(Boolean);244  const trackHeight = Math.round(height * (1 + Math.max(0.5, length)));245 246  const vars = {247    "--mf-sft-h": `${height}px`,248    "--mf-sft-r": `${borderRadius}px`,249    "--mf-sft-dim": dimColor,250    "--mf-sft-fill": fillColor,251    "--mf-sft-fs": `${fontSize}px`,252    "--mf-sft-n": words.length,253    ...style,254  } as React.CSSProperties;255 256  return (257    <div ref={rootRef} style={vars} className={`${styles.root} ${className}`.trim()}>258      <div ref={viewportRef} className={styles.viewport} tabIndex={0} role="region" aria-label="Scroll to reveal text">259        <div className={styles.track} style={{ height: `${trackHeight}px` }}>260          <div className={styles.stage}>261            <div className={styles.inner}>262              {eyebrow && (263                <span aria-hidden="true" className={styles.eyebrow}>264                  {eyebrow}265                </span>266              )}267              <p className={styles.paragraph}>268                <span className={styles.srOnly}>{text}</span>269                <span aria-hidden="true">270                  {words.map((word, i) => (271                    <React.Fragment key={i}>272                      <span className={styles.word} style={{ "--mf-sft-i": i } as React.CSSProperties}>273                        {word}274                      </span>{" "}275                    </React.Fragment>276                  ))}277                </span>278              </p>279            </div>280            <span aria-hidden="true" className={styles.hint}>281              <span className={styles.hintLine} />282            </span>283          </div>284        </div>285      </div>286    </div>287  );288}